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ARROW-7786: [R] Wire up check_metadata in Table.Equals method

In tests, use `expect_equal()` to test that Tables and Schemas are the same, including metadata. Use `expect_equivalent()` to skip the metadata check.

This patch also adds the `Schema$WithMetadata()` method to set key-value metadata. It also fixes a bunch of invalid Equals methods that either errored or segfaulted when you passed a non-Arrow object to them.

I created https://issues.apache.org/jira/browse/ARROW-7891 as a followup for adding a check_metadata argument to RecordBatch->Equals in C++, since it seems like it should exist.

Closes #6459 from nealrichardson/r-table-equals and squashes the following commits:

13d8cea74 <Neal Richardson> Doc and lint
3b1abe4d6 <Neal Richardson> Fix a bunch of segfaults and faulty Equals methods
081abbae9 <Neal Richardson> expect_equivalent does not check metadata
a4a3d9584 <Neal Richardson> Wire up Table__equals(check_metadata). Add bindings/methods to let you set metadata

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>